公式ドキュメントの記載

If you just need to make a one-time request and don't want to leave the thread pool around, you can create the RequestQueue wherever you need it and call stop() on the RequestQueue once your response or error has come back, using the Volley.newRequestQueue() method described in Sending a Simple Request. But the more common use case is to create the RequestQueue as a singleton to keep it running for the lifetime of your app, as described in the next section.